Wierd missfire

My Girlfriend has a Mk1 cab 1.5 JB code and it has developed a wierd missfire.

The car starts and idles fine, accelerates and coasts with my foot off the pedal ok.
Once i have accelerated to say 60 and then reduced the throttle to cruise at a steady speed the missfire starts - the revs instantly drop by around 1000rpm then if i either take my foot of the throttle completely or give it more throttle the revs instantly jump back up to normal.

The plugs, leads, cap, rotor arm and condeser have been changed recently and has also had a new exhaust.

Any ideas?  

Cheers
Paul

Manual or auto?

Manual 4 spd

Sorry - you'll have to ber with me on this:

The car is IN GEAR and the clutch is UP, and the revs DROP 1000rpms?

Do the revs drop, or the rev counter reads 1000rpm down (ie does the engine note change too)?

Sorry,

car is in gear clutch up rev counter drops 1000rpm and there is a noticable judder and engine tone changes - only when a small amount of throttle is applied

Thanks